Novel promoters for the anionic polymerization of lactams



United States Patent 6 3,499,875 NOVEL PROMGTERS FOR THE ANIONIC POLYMEREZATIION F LA'CTAMS Edward W. Pietrusza, Morristown, and Jack R. Pedersen,

Parsippany, N.J., assignors to Allied Chemical Corporation, New York, N.Y., a corporation of New York No Drawing. Filed July 24, 1967, Ser. No. 655,298

int. Cl. C08g 20/10, 20/12, 20/18 US. Cl. 260-78 8 Claims ABSTRACT OF THE DISCLOSURE The anionic polymerization of lactams whereby a lactam is polymerized in the presence of an anionic catalyst is well known. This polymerization has advantages over the more conventional hydrolytic polymerization in that large quantities of diluent are not required. Moreover, the anionic polymerization produces high molecular weight polymer much more rapidly than does the hydrolytic process, on the order of minutes as against many hours to attain similar molecular Weights. However, temperatures of polymerization above the polymer melting point are required to initiate the polymerization. These high temperatures promote rapid polymerization to high molecular weights, but since the equilibrium between monomer and polymer is temperature dependent and high polymerization temperatures result in lowered conversion of monomer to polymer, large quantities of monomer may be present in the anionically polymerized polymer. Since a high monomer content will adversely alfect polymer properties, a purification step is generally required to remove the monomer. Thus low polymerization temperatures are desirable to increase the conversion of monomer to polymer. Temperatures of polymerization below the polymer melting point are also desirable when molded articles are to be prepared, since the lactam can be polymerized directly in the mold which can be of large or intricate design, and solid articles shaped to conform to the mold can be prepared without further processing or machining.

Various cocatalyst promoters have been disclosed in the prior art to increase the rate of polymerization of lactams to high molecular weight polymer at temperatures below the polymer melting point, but the need for new promoters continues.

It is a principal object of the present invention to provide novel cocatalyst promoters for the anionic polymerization of lactams.

It is another object to provide novel compounds useful as cocatalyst promoters for the anionic polymerization of lactams.

It is a further object to provide cocatalyst promoters which make possible the conversion of lactams to high molecular weight polyamides rapidly at temperatures below the polymer melting point.

It is another object to provide an improved process for the polymerization of lactams at temperatures below the polymer melting point.

Further objects will become apparent from the following detailed description thereof.

We have discovered novel cocatalyst promoters for the 'ice anionic polymerization of lactams which effect polymerization of lactams to very high molecular weights rapidly, i.e. in a few minutes, at temperatures well below the polymer melting points. These novel cocatalysts comprise certain benzoxazolyl derivatives. The benzoxazolyl compounds found particularly effective according to the invention include 1- (2-benzoxazolyl benzotriazole di- (2-benzoXazolyl) thioether o \O/ N ll 0 1- (2-benzoxazolyl) benzoxazolinone-Z These compounds can be prepared by condensation of 2-chlorobenzoxazole with a heterocyclic nitrogen compound containing a reactive hydrogen atom. This reaction can be illustrated by the condensation of 2-chlorobenzoxazole and l-benzotriazole as set forth in the equation below:

Typically equimolar amounts of the reactants are dissolved in a common solvent and heated :at a temperature from about 50 C. up to the reflux temperature of the solution until the evolution of hydrogen chloride ceases. Preferably the reaction is carried out under anhydrous conditions.

The solvent must be inert to the reaction and must be a solvent for the reactants. Preferably it will be a solvent for the product as well although this is not required. Suitable solvents include aromatic hydrocarbons such as benzene, toluene, xylene, ethylbenzene, m-diisopropylbenzene, and the like.

The product can be recovered in a conventional manner, as by removal of the solvent or precipitation of the product by addition of a nonsolvent. The product can be further purified by recrystallization from a suitable sol- 3 vent or by fractional distillation as will be known to one skilled in the art.

The cocatalyst promoters described above are effective for the polymerization of lactams having from 6 to 12 carbon atoms in the lactam ring. Suitable lactams include e-caprolactam, enantholactam, caprylolactarn, laurolactam, and the like. These lactams can be homopolymerized or copolymerized with each other.

The anionic catalyst for the polymerization of the lactam is conventional and comprises a metal salt of the lactam. These known anionic catalysts can be prepared by heating an alkali metal, an alkaline earth metal, or a basically reacting compound of such metals including their hydrides, amides, oxides, hydroxides, carbonates, salts of weak acids, metallo-organic compounds, and the like, with excess lactam. The salt-forming metal or metallic compound is admixed with the molten lactam under anhydrous conditions to form a reactive mixture containing from about 0.1 to about 5, preferably from about 0.4 to about 2.0 mols of the metal per 100 mols of lactam. The temperature is maintained above the melting point of the lactam during this step.

The addition of one of the above-described cocatalysts to the lactamlactam salt reaction mixture results in the rapid polymerization of the lactam to the corresponding polyamide. In general, from about 0.1 to about mols of cocatalyst is required for each mol of anionic catalyst present. Since the molecular weight of the polymer decreases as the amount of cocatalyst increases, relatively small amounts, on the order of 0.05 to 1.5 mols per 100 mols of lactam will be employed when high molecular weight polymer is desired. For example, polycaprolactam having high molecular weight, equivalent to reduced viscosities as a 0.5 percent by weight solution in m-cresol at 25 C. of from about 4 to 12, can readily be obtained using from about 0.7 to 1.4 mols of anionic catalyst and from about 0.2 to 0.7 mol of cocatalyst per 100 mols of caprolactam.

Lactams can be anionically polymerized at temperatures from the melting point of the lactam monomer up to the decomposition temperature of the polymer. However, temperatures above the melting point of the lactam monomer but below the flow point of the resultant polyamide are particularly suitable for the preparation of molded articles and are preferred according to the present invention. A suitable polymerization temperature for preparing molded articles from E-caprolactam for example is from about 110 C. to about 215 C., and the preferred range, according to the process of the invention, is from about 150 C. to about 180 C.

It is necessary that the polymerization process as disclosed herein be conducted under substantially anhydrous, nonacidic conditions. Those compounds which are capable of acting as proton donors are to be excluded from the reaction mixture, inasmuch as these compounds readily interact with and decompose the lactam-salt species in the reaction mixture. The presence of a protondonating species, particularly water, should be kept below about 50 p.p.m.

The polymerization process is preferably conducted by adding a cocatalyst promoter of the invention to a reaction mixture containing the desired lactam and a metal salt of the lactam, but a reverse procedure can be utilized if desired, i.e., the cocatalyst can be added to the lactam and the alkali metal or alkaline earth metal or salt-forming compound thereof can be added thereafter. Alternatively, if desired, it is possible to add a cocatalyst promoter of the invention simultaneously with the anionic catalyst to the lactam.

The metal salt of the lactam is preferably prepared in situ immediately prior to its utilization in the polymerization process to minimize risk of contamination. However, a mixture of said metal salt of the lactam and the lactam, particularly when the lactam is E-caprolactarn, is stable at room temperature for a month or more and even at higher temperatures, up to about C., the solution is stable for several days. Accordingly, the salt can be prepared in advance and stored if desired.

By utilization of a cocatalyst of the invention in conjunction with an anionic catalyst, there is obtained a rapid rate of polymerization as well as a high rate of conversion to polymer when temperatures of polymerization below the melting point of the polymers are employed. This eliminates the necessity of removing monomer from the polymer.

Moreover, in accordance with the process of the invention, polycaproamide can be obtained having reduced viscosities as a 0.5 porcent solution in m-cresol at 25 C. of at least 4 corresponding to molecular weights considerably higher than those usually achieved by conventional hydrolytic polymerization processes. Such high molecular weight polymers posses greater tensile strength and toughness than polymers of lower molecular weights, corresponding to reduced viscosities of from about 1 to 2.5, which are generally obtained by conventional processes.

In addition, the polymerization of relatively fluid monomer to solid polymer in our process allows polymerization of e-caprolactam directly in molds including molds of intricate design to form finished solid shaped articles.

A further advantage is that various filler materials, such as sand, pigments, blowing agents, glass or asbestos fibers, low-density fillers, heat and light stabilizers, and the like, nucleating agents such as boron-nitride, magnesium silicate, etc., and plasticizers if desired, can be incorporated in the polymer readily and conveniently by admixing with the monomeric lactam, in a homogeneous manner or otherwse, just prior to polymerization.

The invention can be illustrated further by the following examples, but it is to be understood that the invention is not meant to be limited to the details described therein.

In the examples, all parts are by weight unless otherwise noted. Reduced viscosity was determined as a 0.5 percent by weight solution in m-cresol at 25 C.

EXAMPLE 1 153 parts of 2-chlorobenzoaxazole and 119 parts of 1- benzotriazole were added to 1000 parts by volume of xylene and the mixture heated at reflux temperature for one hour. The evolution of hydrogen chloride was noted. A crystalline product precipitated in cooling which was filtered off and dried. 210 parts of the product were obtained, corresponding to an 89% yield.

The structure for the compound l-(2-benzoxazolyl)- benzotriazole Q CN/ EXAMPLE 2 78 parts of 2-chlorobenzoxazole and 76 parts of 2- mercaptobenzoxazole were added to 1000 parts by volume of xylene and heated at refiux for about five hours until evolution of hydrogen chloride ceased. A portion of the solvent was distilled otf and the solution allowed to cool. 110 parts of a product having a melting point of 55-56" C. was obtained.

Elemental analysis was as followsCalculated for C H N O S: C, 62.7; H, 3.0; N, 10.4. Found: C, 62.7; H, 3.1; N, 10.2.

The structure of di-(Z-benzoxazolyl)thioether was confirmed by infrared analysis.

200 parts of e-caprolactam and 1.4 parts of lithium hydride were charged to a tube as in Example 1. When evolution of hydrogen ceased, 1.4 parts of di-(Z-benzoxazolyl)thioether as prepared above was added. The mixture became viscous after one minute at 160 C. and solidified after eleven minutes. A yellow, hard, brittle, insoluble polymer was obtained.

EXAMPLE 3 154 parts of 2-chlorobenzoxazole and 120 parts of 2- benzoxazolinone were added to 500 parts by volume of xylene and the mixture heated at reflux for seventeen hours. Upon cooling, a precipitate formed and was collected. After recrystallization from xylene a 77.5% yield of product having a melting point of 182-l85 C. was obtained.

Elemental analysis was as follows-Calculated for C H O N C, 66.7; H, 3.2; N, 11.1. Found: C, 66.5; H, 3.2; N, 11.1. The structure for 1-(2-benzoxazolyl)- benzoxazolinone-2 was confirmed by infrared analysis.

200 parts of e-caprolactam and 2.0 parts of lithium hydride were reacted at 160 C. as in Example 1 and and 1.3 parts of 1-(2'-benzoxazolyl)benzoxazolinone-2 as prepared above were added. After one minute the mixture became viscous and it solidified after seven minutes. The polymer was heated for an additional thirty minutes at 160 C.

The reduced viscosity of the polymer product was 6.7.

EXAMPLE 4 154 parts of 2-chlorobenzoxazole and 96 parts of 3,5-dimethylpyrazole were added to 1000 parts of volume of xylene. The mixture was heated at reflux for three hours. A precipitate formed on cooling which was collected by filtration and recrystallized from n-heptane. 146 parts of a product having a melting point of 121- 122 C. was obtained. Solvent was distilled from the filtrate and the residue recrystallized from n-hetptane. An additional 35 parts of product was obtained, so that the total yield was 84.6%.

Elemental analysis was as follows-Calculated for C H N O: C, 67.6; H, 5.2; N, 19.7. Found: C, 67.6; H, 5.2; N, 19.6.

The structure of the product, l-(2'-benzoxazolyl)-3,5- dimethylpyrazole (ID-C H3O was confirmed by infrared analysis.

200 parts of E-caprolactam and 1.4 parts of lithium hydride were reacted at C. as in Example 1. 1.25 parts of 1-(2-benzoxazolyl)-3,5-dimethylpyrazole as prepared above were added. The mixture became viscous after one minute, and it solidified after five minutes. The polymer was heated at 160 C. for an additional fifteen minutes.

The off-white, hard, tough polymer had a reduced viscosity of 7.2.

EXAMPLE 5 400 parts of e-caprolactam and 2.8 parts of lithium hydride were heated at 160 C. in a tube as in Example 1. After two hours the mixture was slightly viscous. The mixture was heated for an hour longer, and one-half of the mixture was poured into water. The solid was filtered, washed, and dried. The reduced viscosity of the polymer was 1.4.

1.3 parts of 1-(2-benzoxazolyl)benzotriazole as prepared in Example 1 were added to the remaining mixture in the tube. It solidified in fourteen minutes. The polymer was heated for fifteen minutes longer to yield an ofi-white, hard, tough polymer having a reduced viscosity of 4.3.

It will be apparent that many modifications and variations can be elfected without departing from the novel concepts of the present invention, and the illustrative details disclosed therein are not to be construed as imposing undue limitations on the invention.

We claim:

1. In a process for the polymerization of a lactam of 6 to 12 carbon atoms in the lactam ring under anionic conditions wherein a lactam salt of a metal of the group consisting of alkali metals and alkaline earth metals is a catalyst, the improvement which comprises adding to the reaction mixture as cocatalyst a member of the group consisting of 1 (2 benzoxazolyl)benzotriazole, di-(2- benzoxazolyl)thioether, 1 (2' benzoxazolyl) 3,5 dimethylpyrazole, and 1 (2 benzoxazolyl)-benzoxazolinone-2 and maintaining the reaction mixture at a temperature from the melting point of the lactam to the softening point of the resultant polyamide.

2. A process according to claim 1 wherein the reaction mixture is maintained at a temperature from the melting point of the lactam to the softening point of the resultant polyamide.

3. A process according to claim 1 wherein the catalyst to cocatalyst mol ratio is from about 120.1 to about 1:10.

4. A process according to claim 1 wherein the lactam to cocatalyst mol ratio is from about 10020.05 to about 5. A process according to claim 3 wherein the lactam is e-caprolactam.

6. A process according to claim 4 wherein the lactam is e-caprolactam.

7. A process according to claim 5 wherein the reaction mixture is maintained at a temperature from about 150 C. up to about C.

8. A process according to claim 6 wherein the reaction mixture is maintained at a temperature from about 150 C. up to about 180 C.

References Cited UNITED STATES PATENTS 3,251,799 5/1966 Pietrusza et al. 260-78 HAROLD D. ANDERSON, Primary Examiner U.S. Cl. X.R. 260-37, 307 

